Michael Williamson, former NSW State secretary of the NSW branch of the Health Services Union, tries to hid in a police car outside Maroubra Police Station this morning. Picture: John Grainger Source: The Daily Telegraph

EMBATTLED MP Craig Thomson says the charges laid against former Health Services Union boss Michael Williamson have nothing to do with him.

Mr Williamson, 59, from Sydney's east, was arrested when he arrived by appointment at Maroubra police station.

Police are expected to lay more charges against Mr Williamson and others after fraud squad detectives charged him with 20 offences today.

Police raided his home and seized computer equipment in the latest development in the ongoing fraud investigation into the union.

That investigation has probed allegations that Mr Williamson and Mr Thomson, the former HSU national secretary, received secret commissions from a union supplier.

Mr Thomson, who has been suspended from the Labor Party, told AAP that Williamson's arrest didn't come as a surprise.

He would not confirm whether Craig Thomson was one of those five but said Mr Williamson's son was.

HSU national secretary Kathy Jackson this afternoon said she is confident she won't face fraud charges related to issues surrounding the union.

Ms Jackson - who helped blow the whistle on alleged misuse of HSU member funds, leading to two Fair Work Australia investigations - is the subject of several complaints by the union, which is seeking to remove her from her position.

Ms Jackson has dismissed rumours she's also been charged as malicious and totally unfounded.

NSW police have been investigating claims of systemic corruption at the HSU East branch, of which Mr Williamson was secretary for 15 years.

It is alleged by police he hindered investigations into fraud against and corruption within the HSU.

Williamson was inside the police station for about an hour this morning before being driven out the back exit by detectives.

When the car was surrounded by the media, Williamson, who was wearing a suit and tie and sitting in the rear of the vehicle, covered his face with his hands.

Detectives anticipate charging him with a number of offences relating to hindering investigations, including recruiting a person to engage in criminal activity and officer of an organisation publishing false statements.

Mr Thomson, who suspended his membership of the ALP earlier this year and now sits as an independent, was assistant secretary of the NSW branch of the HSU from 1999 to 2002.

He became HSU national secretary before entering parliament in 2007, was accused of misusing more than $500,000 of union member funds, including on prostitutes. He denies all wrongdoing.

Opposition Leader Tony Abbott said Australians would be shocked Mr Williamson had been arrested in Sydney and was facing possible criminal charges.

"Plainly the prime minister has got to take responsibility for this,'' Mr Abbott told reporters in Melbourne.\

Ms Gillard was the last person in the country to express full confidence in Mr Thomson, he said.

"The prime minister is still relying on the tainted vote of the Health Services Union representative in the federal parliament.''

Mr Abbott said until Ms Gillard completely repudiated the type of union activity alleged at the HSU, the public would conclude the government was ``seriously ethically challenged'' and that ''...this is a prime minister who lacks judgement as well as lacking competence.''

Ms Gillard would not be drawn on the matter.

"I'm not going to comment on a police matter,'' she told reporters in Hobart.

"I don't want to be speaking about a matter which will ultimately be dealt with by the courts.''

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